Three main objectives of the project are: using our isolated perfused working rate heart preparation as studied by P-31 NMR we will do the following: a) Measurement of levels of ATP, CrP and Pi in the steady state as a function of developed pressure caused by variation of both preload and afterload. b) Measurement of changes of ATP, CrP and Pi at various positions in the cardiac cycle as a function of developed pressure and carbon substrate availability (glucose vs glucose plus pyruvate). c) Useof saturation transfer P-31 NMR to measure unidirectional and net flux values of high-energy phosphate through the creatine kinase reaction in isolated perfused working rat hearts. d) Make saturation transfer measurements as a function of these same parameters. Further, these saturation transfer measurements as well as the newly implemented inversion transfer measurements will be gated to the cardiac cycle in order to determine whether the rates of the creatine kinase reaction are constant or change during the cardiac cycle.